The present invention relates to a digital modulation method employing digital signals having a word length of k bits (k=1, 2, 3, . . . ) according to which modulation signals having certain amplitudes and phase positions are associated with the 2.sup.k possible bit words, these modulation signals being formed by a combination of three carrier waves which are shifted in phase by 120.degree. relative to one another and have a constant frequency and weighted amplitudes.
Such a digital modulation method is disclosed in U.S. Pat. No. 3,805,191. This patent discloses, in particular, a method for modulating a carrier signal with higher valued digital signals, i.e. with digital signals whose bit word length is greater than 2. The object is there to modulate the carrier signal in such a manner that it is as unsusceptible as possible to interference in the transmission path in order to be able to recover in the receiver the original data signal from the modulated carrier signal without errors.
If the digital signal has a word length of 2 or perhaps even 3 bits, it is sufficient to only phase modulate the carrier signal with a constant amplitude. Pure phase modulation soon reaches its limits since with an increasing number of possible bit words the phase difference between the signals at adjacent phase positions decreases. When such phase difference becomes too small, there easily exists the danger that even a slight fluctuation in phase will cause a particular signal to be evaluated as belonging to the adjacent signal phase.
For that reason, and in order to realize a modulation as insensitive to interference as possible with higher valued data signals, use is made of phase-amplitude modulation, i.e. the signals associated with each bit word have a particular phase and amplitude. The selection of the phase and amplitude differences between different signals should now be made in such a manner that the usual interference occurring during signal transmission, such as noise, phase shifts, oblique amplitude positions, multiple path propagation and the like, will not interfere with the signal amplitude and phases to such an extent as to cause incorrect signal evaluations. The above-mentioned U.S. Pat. No. 3,805,191 discloses a modulation method based on phase-amplitude modulation, which has a greater signal to noise ratio than conventional methods (e.g. quadrature amplitude modulation QAM). In contradistinction to QAM, in which the various signal states are produced by the bit word controlled combination of two 90.degree. phase shifted carrier signals with their amplitudes correspondingly weighted, the method disclosed in U.S. Pat. No. 3,805,191 combines three 120.degree. phase shifted and amplitude weighted carrier oscillations; i.e., according to this method, the signal vectors are selected with respect to their amplitude and phase so that their ends lie in the centers of identically sized, regular hexagons which are arranged in the polar coordinate amplitude-phase plane in a honeycomb pattern without interstices around the vector origin M (see FIG. 1).
It can be seen from the phase plane shown in FIG. 1 that the angles between some pairs of signal vectors, e.g. between vectors 2 and 11 or 5 and 15, etc. are rather small, so that it is easily possible for signals associated with these vectors to be evaluated incorrectly due to phase shifts. With respect to phase shifts, this modulation method is thus still rather subject to interference. Moreover, the irregular angle distribution produced by the vectors in the phase plane is unfavorable for demodulation since the demodulator must be able to precisely recognize several very different phase states. If thus the unequivocal determination of the phase states is made more difficult, the error probability during demodulation is increased.